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Scaphoid Non union Advanced Collapse (SNAC)

Doctor Attia

Scaphoid Non union Advanced Collapse (SNAC)

AP and lateral views X ray of the wrist joint of 34 years old male patient showing SNAC

Definition 

  • Scaphoid Non union Advanced Collapse (SNAC) is defined as progressive arthritis of the wrist joint that results from a long standing scaphoid nonunion.

  • It is like the same sequela of scapholunate dissociation as it progresses to scapholunate advanced collapse (SLAC).


Pathology 

  • In a SNAC wrist, the proximal scaphoid fragment usually rotates with the lunate during extension, while the distal scaphoid fragment rotates into flexion.

  •  This abnormal movement results in early osteoarthritis between the distal scaphoid fragment and the radial styloid process.


Stages 

4 SNAC stages of osteoarthritis (between 2 structures:

  • stage 1

 the distal scaphoid fragment and the radial styloid.

  • stage 2

 stage 1 + the scaphoid and the capitate

  • stage 3

stage 1+2+ the lunate and the capitate + proximal radial migration of the capitate dorsal to the lunate (DISI).

(periscaphoid arthrosis) 


Blood supply of scaphoid

1- Dorsal carpal branch (which is a branch of the radial artery) 

- the major blood supply.

- enters scaphoid in the dorsal surface 

- supplies proximal 80% of scaphoid 

- retrograde blood flow

2- Superficial palmar arch (which is a branch of volar radial artery)

- minor blood supply 

- enters distal tubercle 

- supplies distal 20% of scaphoid


Diagnosis

SNAC diagnosis is made clinically with:


Symptoms of SNAC

  • progressive wrist pain 

  • wrist instability 

  • weakness

  • reduced grip 

  • reduced pinch strength

  • stiffness with extension and radial deviation


Clinical examination of SNAC

  • tenderness of the radioscaphoid articulation

  • decreased wrist motion on extension and radial deviation



 Radiographs

showing scaphoid fracture nonunion with advanced arthritis as described in stages 


Treatment 

Nonoperative

in low functioning or unfit patients only


Surgical Treatment of SNAC

Stage I:

  • radial styloid excision + bone graft of scaphoid nonunion

Stage II: 

  • proximal scaphoid fused to capitate and lunate with excision of distal scaphoid

  • scaphoid excision, four-corner fusion

Stage III: 

  • scaphoid excision, four corner fusion

  •  wrist arthrodesis


Important Notes

  • Bleeding of bone during surgery is a very good prognostic indicator of possible union.

  • Midcarpal fusion with scaphoid excision provides fair motion and strength equal to complete fusion, but wit risk of arthritis progression in some cases.

  • Proximal row carpectomy provides more motion, a little less strength, and progression of arthritis in some cases.

  • Total wrist arthrodesis is the end stage and provides good strength, but with no motion.



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